The present invention relates generally to electrolytic cells, and particularly cells where chlorine gas is reduced at the cathode electrode and chloride ions are oxidized at the anode electrode.
One known application for such a cell, also referred to as a chlorine-chlorine cell, is the separation of chlorine gas from a stream of chlorine and foreign gases. Such foreigh gases could include, but are not limited to, carbon dioxide, oxygen and hydrogen gases. Although the chlorine-chlorine cell separation technique could be useful in the manufacture of chlorine gas, the principal application herein relates to zinc-halogen batteries, such as a zinc chlorine battery. In the zinc-chlorine battery application, the foreign gases are also referred to as inert gases. This is because these gases are inert in the hydrate formation process whereby chlorine is stored in the battery. During the charging of a zinc-chlorine battery, chlorine gas is evolved at the positive electrode (anode) and zine metal is deposited on the negative electrode (cathode). Thus, inside the battery casing, the environment is necessarily a chlorine gas environment. However, small quantities of other gases may also be present inside the battery case. For instance, carbon dioxide is evolved during normal operation of the battery as a by-product of the oxidation of the battery graphite. The volumetric rate of carbon dioxide evolution during battery charging is approximately 0.02% to 0.04% of the chlorine gas evolution rate. Consequently, if the carbon dioxide is not purged from the battery system, it will accumulate over a period of charge/discharge cycles, and eventually interfere with the normal operation of the battery.
Two known cells of this type are described in a co-filed U.S. Patent Application entitled "An Electrolytic Cell For Separating Chlorine Gas From Other Gases", assigned to the assignee of the present invention. The first cell was comprised of two porous (gas permeable) graphite electrodes, separated by an asbestos membrane saturated with an aqueous zinc-chlorine electrolyte. The chlorine and foreign gases were passed into the cell through the cathode electrode, and the purified chlorine gas was passed out of the cell through the anode electrode. The foreign gases and unreacted chlorine gas were vented from the gap between the cathode electrode and asbestos membrane at the top of the cell. This early proof of principal cell provided a 50%-60% recovery of chlorine gas from the input gas mixture. Thus, approximately one half of the chlorine gas entering the cell was being vented with the foreign gases. A discussion of this cell and the zinc-chlorine battery application may also be found in: Development of the Zinc-Chlorine Battery for Utility Applications, Interim Report, April 1979, pages 36-9, 12, published by the Electric Power Research Institute, Palo Alto, Calif., and is herein incorporated by reference.
The second cell was a cylindrical design, where the anode electrode was a cylinder and the cathode electrode was a rod coaxially disposed in the anode. Both the anode and the cathode electrodes were made from dense graphite (gas and liquid impermeable). The concent of a cathode assembly was also introduced with this cell design. The cathode assembly was comprised of the cathode electrode, a porous graphite (gas impermeable, but liquid permeable) membrane cylinder interposed between the anode and cathode electrodes, and a quantity of graphite powder packed between the membrane cylinder and the cathode electrode. Thus, the cathode electrode rod was coaxially disposed in the membrane cylinder, and the membrane cylinder was coaxially disposed within the anode electrode cylinder. The cathode assembly had a gas inlet (for the chlorine and foreign gases) at the bottom of the cell, and a gas outlet (to vent the foreign and unreacted chlorine gases) at the top of the cell. However, the cathode was otherwise sealed to prevent the foreign gases from mixing with the purified chlorine gas generated at the anode. The electrolyte was an aqueous hydrochloric acid electrolyte.
Another known cell, herein referred to as the third cell, was generally comprised of two flat-plate anode electrodes and a common cathode assembly interposed between them. As in the second cell design, the electrodes were made from dense graphite. The cathode assembly was comprised of a flat-plate cathode electrode, a flat-plate porous graphite membrane disposed on each side of the cathode electrode, and a graphite powder packing between the membranes and the cathode electrode. A housing was used to align the electrodes and membranes in parallel, and also to provide a means for sealing the cathode assembly.
Although both the second and third cells provided a higher efficiency (more chlorine gas reacted per energy input) than the first cell, they also suffered some drawbacks. The most important of these drawbacks related to the third cell design, where within a short time the stream of chlorine and foreign gases woule cause passageways to form in the packed bed of graphite particles in the cathode assembly. Such gas passageways are undesirable for two reasons. First, as the graphite particles form the primary surface or sites for the reduction of chlorine gas, inactive areas or dead spots would be formed in the cathode assembly. Second, the gas passageways would naturally seek a path for the gas outlet at the top of the cathode assembly, and thus increase the amount of chlorine gas being vented from the cell with the foreign gases. These gas passageways were particularly detrimental to the third cell design, because the gas passageways would preferentially form on only one side of the cathode electrode. This resulted in the major portion of the input gas mixture flowing into the side of the cathode assembly with the gas passageways, and essentially starving the other side of gas flow.
Another drawback of these cells relates to the cathode assembly seal. In these cells, the housing was used in part to seal the cathode assembly at the top and bottom of the cell. The difficulty with this approach was in attaining a gastight seal between the plastic or lucite housing and the porous graphite membrane(s). If a gas tight seal is not achieved, the purpose of the cell is defeated because the foreign gases will diffuse into the anode section of the cell and mix with the purified chlorine gas being generated.
A further drawback relates to the rate at which the input gas mixture must be processed through the cell and also the size of the cell. When the gas flow rate into the cell is very low, even an inefficient cell will be capable of reducing all or substantially all of the chlorine gas at the cathode. This is especially true if the applied voltage across the cell is relatively high (i.e. about two volts), as it will keep the cathode very cathodic. However, when the gas flow rate is increased significantly, even an efficient cell may not be capable of reducing all of the chlorine gas. This results in unreacted chlorine gas being vented from the cathode assembly along with the foreign gases. This result is unacceptable because it is desirable to vent the foreign gases into the atmosphere. Thus, with relatively high gas flow rates it is a practical necessity to have more than one cell or stages in order to handle any overflow of unreacted chlorine gas from the cell. The subsequent cell would use as its input the outlet from the cathode assembly of the previous cell. In such a situation it would be important to have a cell design which would provide for the addition of one or more subsequent cells or stages in a readily manufacturable and economical manner. In reference to the zinc-chlorine battery application, there is the further constraint of the size of the cell(s). As it is desirable to locate the cell(s) inside the battery casing, the size of the cell(s) must be kept to a minimum. Since the second and third cell designs suffer inefficiencies at higher gas flow rates, subsequent cells or stages would be needed to insure a total removal of chlorine gas from the foreign gas vent than would be required in a more efficient cell design. However, these cell designs do not provide a compact structure when combined with further cells.
The present invention provides a cell design which substantially minimizes or eliminates the formation of gas passageways in the graphite packed bed, achieves an integral seal for the cathode assembly, and is readily adaptable to a compact multistage structure. Particularly, the present invention provides for a novel cathode assembly. The cathode assembly is generally comprised of a cathode electrode plate having a winding channel means formed in the face opposing the anode for directing the flow of gas through the cathode assembly, a porous graphite membrane sealed into the face of the cathode electrode over the channel means, and a packing of graphite particles contained in the channel means. The cathode assembly also includes an inlet port at the beginning of the channel means for receiving the chlorine and foreign gases, and an outlet port at the end of the channel means for venting the foreign gases from the cell. The anode is a flat-plate electrode made from dense graphite. A chemically inert non-conductive housing is used to separate the anode and cathode assembly a predetermined distance, and align them in parallel. The housing further serves to contain the electrolyte and hermetically seal the cell. The aqueous electrolyte is composed of dilute hydrochloric acid, preferably between 5% and 15% of the electrolyte by weight. A direct-current constant-voltage power supply is used to provide a potential difference across the anode and cathode electrodes sufficient to cause the chlorine gas reduction at the cathode and the chloride ion oxidation at the anode. Alternatively, the housing may be replaced by a non-conductive separator member interposed between the anode electrode and the cathode assembly, and means for sealing the anode electrode and cathode assembly to the separator member.
The present invention further provides for a novel multi-stage cell design. The multi-stage electrolytic cell is generally comprised of a single anode electrode and cathode assembly contained in a housing with an aqueous hydrochloric acid electrolyte. The cathode assembly is comprised of a single cathode electrode plate having a plurality of adjacent channel means formed in the face opposing the anode electrode, a corresponding plurality porous graphite membranes sealed into the face of the cathode electrode over the channel means, and a packing of graphite particles contained in each of the channel means. The cathode assembly further includes passage means formed in the face of the cathode electrode opposite to the channel means for connecting the end of the first or previous channel means with the beginning of the second or subsequent channel means.